Heavy oil and bitumen deposits are found in many areas of the world, including Alaska, Canada, Siberia and the Nordic countries. Typically, deposits of heavy oil are mixed with deposits of lighter oil. Enhanced recovery of the heavy oil deposits generally require a reduction in viscosity, which refers to the propensity of a fluid to flow. Thus, in order to recover heavy oil, it must first be processed in some manner to reduce its viscosity to allow the oil to flow.
Currently, viscosity reduction is often accomplished either by: (1) increasing oil temperature through (a) injecting steam with or without gaseous additives such as methane, propane, natural gas, nitrogen, or CO2, or (b) in-situ combustion through injecting oxygen-containing gases such as air; or (2) dilution of the oil through injecting low-viscosity hydrocarbon solvents. Enhanced oil recovery (EOR) on the North Slope of Alaska and similar locations is usually done with the use of heated steam injection. Heavy oils have such high viscosity at reservoir conditions that recovery rates by typical EOR methods are much lower than for lighter oils. Thus, the recovery of heavy oils has marginal economics or is uneconomical altogether. Challenges exist with EOR methods even when the oil reaches the well bore and production facilities because of asphaltic precipitation and the formation of emulsions. Although current technology is improving, the recovery of heavy oil remains costly to the point that many deposits are not economic to produce.
Today, long-reach, multilateral drilling techniques developed during the last 10 years are improving the economics of heavy oil production in areas such as the North Slope. At present, four percent of the oil in the trans-Alaska oil pipeline, some 35,000 barrels per day, is now heavy oil. This heavy oil is recovered using primarily CO2 injection and gravity flow from horizontal production wells at formation depths of up to several thousand feet where formation temperatures range from around 70 degrees Fahrenheit to less than 100 degrees Fahrenheit. The heavy oil resource just on the North Slope is huge: the 20-25 billion barrels of heavy oil in place is more than was present at Prudhoe Bay before production started. While North Slope natural gas is the undeveloped resource that gets most of the attention from producers, there is actually more resource in heavy oil than there is in gas in Prudhoe, Point Thomson, and all the other gas discoveries on the North Slope in the greater Prudhoe Bay area.
Since CO2 commonly occurs with natural gas, including as a component of natural gas, frequently there is an inexpensive way of obtaining the CO2 that is needed to mobilize the oil. However, in many areas this supply is dwindling and now requires the piping of CO2 from outside many of the fields to be used as a mobilizer for the oil. The production of steam to be used to mobilize the oil is also somewhat inefficient because of the energy that is necessary to create the heat to heat the water to produce steam is generally a poor substitute for CO2. Since both processes have an inherent dependence upon the pathway through the formation to get to the heavy oil to be transported, there is a great deal of difficulty in focusing the solutions on the heavy oil to be transported. The heavy oil will either block the pore spaces or, as a result of some of the heavy oil being mobilized, create a channeling effect in the formation. In addition, with pressure buildup when forcing either CO2 or steam into the formation containing the oil there is a great deal of potential that the formation will be fractured and thereby communication from the injection and recovery wells could be lost.
Accordingly, there is a need to alleviate the complications associated with tertiary recovery methods of heavy oil using CO2 and steam.